1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission which has a plurality of hydraulic engaging elements such as clutches and brakes.
2. Description of Related Art
A well-known version of this control apparatus is described in Japanese Published Unexamined Patent Application No. 303349/1989. Namely, in an oil passage which leads to a hydraulic engaging element for a low-speed transmission train, there is disposed an orifice as well as a check valve for oil discharging which is in parallel with the orifice. There is also provided a bypass passage Which bypasses the orifice and in this bypass passage there is interposed a gate valve to open and close it. At the time of changing over from a neutral range to a running range (i.e., at an in-gear time or at the time of gear engagement), the gate valve is opened under a predetermined condition.
In the above-described prior art, its object lies in alleviating the in-gear shocks (i.e., shocks at the time of gear engagement) by adjusting the speed of hydraulic oil supply to the hydraulic engaging element for the low-speed transmission train at the time of gear engagement. It does not however take into consideration at all the off-gear shocks (i.e., shocks at the time of gear disengagement) which occur at the time of switching from the running range to the neutral range (i.e., at the time of gear disengagement).
Off-gear shocks are the shocks that occur in the following manner. Namely, when the vehicle is stopped in the running range, the driving unit made up of an engine and a transmission is displaced in position while elastically deforming engine mounting elements by the driving reaction forces due to the power transmission via the low-speed transmission train and then, at the time of changing over to the neutral range, the driving unit returns from the displaced state to the normal position, at which time the off-gear shocks occur. In the above-described prior art, the off-gear shocks occur on the following occasion. Namely, at the time of gear disengagement, the hydraulic oil pressure in the hydraulic engaging element for the low-speed transmission train rapidly drops by oil discharge via the check valve that is in parallel with the orifice. By the rapid disengagement of the low-speed transmission train, the driving unit suddenly returns to the normal position, giving rise to the occurrence of the off-gear shocks.
Here, if the hydraulic oil pressure in the hydraulic engaging element for the low-speed transmission train is slowly decreased by means of the orifice that is disposed in the oil passage thereof, the off-gear shocks may be alleviated. However, in case the viscosity of the oil is high at a low temperature, the disengagement of the hydraulic engaging element for the low-speed transmission train will be delayed too much. There will consequently occur a dragging of the hydraulic engaging element, accompanied by the deterioration in durability. In addition, at the time of gear disengagement when the accelerator pedal is depressed or when the vehicle is running, there will also occur deterioration in durability caused by dragging unless the hydraulic engaging element for the low-speed transmission train is disengaged quickly.
In view of the above points, the present invention has an object of providing a control apparatus in which the off-gear shocks are alleviated and in which the impairing of the durability due to the dragging of the hydraulic engaging element for the low-speed transmission train can be prevented.